Feeding and reproductive ecology of breeding barn swallows

In the breeding season, barn swallows depend on favourable feeding conditions, since the energy requirements of the growing nestlings are considerable. We study the influence of weather and habitat conditions on the aerial food supply. Furthermore, we record the food quantity provided by the parents, changes in parental body mass and patterns of nestling growth.

Aims

We investigate the feeding ecology of the barn swallow, a characteristic bird species of agricultural landscapes. We document the strategies adopted by the swallows to optimize costs and benefits of foraging in different agricultural field types and under different weather conditions. We aim to find out how parents maximize their annual reproductive output of viable offspring.

Approach

The study area in the plain of Wauwil (canton Lucerne) is a characteristic agricultural landscape in central Switzerland. From 1998-2004, we surveyed breeding biology, feeding ecology and growth of nestlings in a population of 100 pairs of barn swallows breeding in 60 farms. In addition, we quantified food availability (i.e. the density of airborne flying insects) in different agricultural fields and under variable weather conditions.

Significance

Environmental conditions affecting food supply are key factors shaping the reproductive output and hence the population dynamics of birds and other animals. Under adverse feeding conditions, breeding adults face a trade-off between self-maintenance and foraging for their brood. To gain insight into the consequences of environmental effects on productivity, it is important to understand (1) the causes and patterns of variation in the food available, and (2) the effects of fluctuating food conditions on the feeding behaviour of parents and the development and survival of their brood.

Results

Broods of barn swallows daily receive 250-350 food portions, each containing 15-20 insects. From hatching to fledging, parental feeding adds up to 100´000 flying insects, mainly flies and midges (Diptera), around one kilogram of insect food. The parents can only collect this enormous amount of insect food, if there are high quality food patches with large numbers of flying insects near the nest. Our counts showed peaks in aerial insect density around hedgerows, in orchards, beneath single trees, on pastures with grazing cattle and above the water of rivers and ponds. At such sites, barn swallows encountered nearly three times more flying insects than when foraging above grassland or arable fields.

In rainy, cold and windy weather, when aerial food supply dropped to very low numbers, there were still some insects in flight around trees and shrubs or inside barns with cattle. Such "weatherproof" food resources function as risk insurance, dampening the effects of generally poor conditions. However, in adverse weather, barn swallow parents only fed half the portions to their nestlings.

Energy demands of broods increased with the number and age of nestlings. Parents adjusted their efforts accordingly, but not when airborne insects were missing. Thus, parents seem to hold their foraging efforts constant, but when prey is scarce, they catch less food for the brood. The nestlings therefore have to survive on their fat reserves, which last for 2-3 days with very little food. At the same time, nestling growth and activity are cut back to a minimum. However, when adverse conditions continue, some nestlings starve, as in July 2001, when over 40% of broods died in a ten-day-spell of cold weather.